1. Field of the Invention
The present invention pertains to the art of welding and is specifically concerned with an apparatus for monitoring the process of resistance flash butt welding.
The invention may find application in machines for continuous resistance flash butt welding mainly of critical workpieces, such as rails and pipes, where higher demands upon the welding quality are placed.
2. Description of the Prior Art
Used as the criterion for evaluating the quality of resistance butt welded joints most often is the conformity of the welding variables with the preset values. The most extensive application has been gained by apparatus with whose aid the welding variables are recorded on a recorder chart and subsequently compared by the operator against a reference. The accuracy of such a control method is largely dependent on a subjective evaluation of the variables by an operator and, because of a slow response of a recorder, fails to reveal short-time interruptions in the process before the upsetting.
Among known apparatus for monitoring the resistance flash butt welding process, best results are offered by such devices which automatically record deviations of the welding variables from specified values (see, e.g., U.S. Pat No. 3,151,290; USSR Inventor's Certificate No. 465,292).
Such apparatus comprise a current transformer, which serves as a current transmitter, a transmitter of the duration of welding circuit short-circuits with a memory element at the output, and an indicating and signalling device which indicates an excess in the duration of the short-circuit before the upsetting of the preset value.
The above-described apparatus, however, monitors the welding process only at its final stage and is incapable of giving complete information on the conformity of the actual values of welding variables with the program of their variation at different stages of fusion. Besides, such apparatus retains the information regarding deviation from the preset welding conditions (going-on of pilot lamps) only till the next welding operation. Therefore, in welding critical workpieces, such as pipings, rails, etc., the use of such apparatus cannot provide a basis for the issuing of a certificate for a welded joint in the form of a digital record of welding process variables.
On the other hand, it should be noted that, while being greatly complex, the construction of modern resistance butt welding machines makes it possible at the same time to derive, either indirectly or directly, various information on the welding process variables. For example, pipes and rails are welded in a machine which has clamps wherein the workpieces to be joined together are installed. One of the clamps is fixed to a movable column which in the course of welding is moved towards a stationary column to bring together the workpieces being weld-joined. The movement of the movable column is controlled by an electrohydraulic actuator with a servo-operated control valve whose rod moves relative to the valve housing. The machine has also other actuators and mechanisms, such as for horizontal and vertical alignment of the workpieces being welded, for removing the metal squeezed out in upsetting, on completion of welding (flash trimmer), etc. The machine incorporates a welding transformer whose primary circuit comprises controlled rectifiers and a current transformer. The above-described machine will hereinafter be referred to as "the machine of said type". In addition, the machine of said type includes an apparatus for monitoring the welding process, which comprises transmitters of the variables being monitored, the transmitters' outputs being coupled to a digital recorder. The transmitters include an analog welding current transmitter coupled to the current transformer's measuring winding, an analog voltage transmitter coupled to the welding transformer primary, an analog transmitter of movable column movement, which may take the form of, e.g., a linear potentiometer, a fusion rate transmitter coupled to armature terminals of the motor of the servo actuator of the moving column, a digital transmitter of welding circuit short-circuit duration with a memory element at the output, coupled to the current transformer, a digital transmitter of offset of the servo-operated control valve rod with respect to the valve housing, as well as digital transmitters of slip of the workpieces being welded, installed in the clamps of both columns.
A programming device employed in this machine comprises a shaper of pulses synchronized with the power supply voltage, a divider of the frequency of the pulses, a welding duration counter, a decoder, a welding program selection unit, a welding initiation flip-flop controlled from a pushbutton, and a set of output flip-flops intended to deliver commands to the machine's actuators mentioned above.
The input of such a programming device is connected to the power mains; the output of the welding duration counter, denoted for the purpose of the present description as the first output, serves to deliver a digital signal which characterizes the time elapsed from the initial moment of welding; one of outputs of the set of output flip-flops, denoted as a second output, serves to deliver a command for changing the effective voltage across the welding transformer primary. The plurality of the remaining outputs of the set of output flip-flops are all designated as a third output; they serve to deliver commands to the above-mentioned actuators of a machine of said type. The output of the welding program selection unit is hereinbelow referred to as the fourth output of the programming device.
The digital recorder includes a commutator of analog signals incoming from the transmitters, an analog-to-digital converter, an interface, a microprocessor with a keyboard entry of a program, a transcriber, and a digital printer.
The apparatus for monitoring the continuous resistance flash butt welding process in the machine of said type functions as follows.
A program for processing the information incoming from the transmitters is entered beforehand into the microprocessor. The pushbutton is depressed to actuate the welding initiation flip-flop in the programming device, and voltage is applied to the welding transformer and to the electrohydraulic actuator of the movable column. A signal for starting the monitoring apparatus is produced simultaneously. A first interrogation of transmitters is carried out, and from its results the information on the set values of the initial voltage at the welding transformer, the no-load current of the welding transformer, the initial rate of fusion, and the initial position of the movable column is printed out. Further interrogations or scannings of the transmitters in the course of welding result in printing out the running value of the welding process variables being monitored.
The number of measurements per a welding cycle is selected depending on the complexity of the program of variation of the process variables. In the prior art apparatus, recording the welding variables is limited to measuring only analog signals, which is insufficient for evaluating the accuracy of implementing the preset welding conditions. For example, in the event of slip of the workpieces in the machine clamps, the linear amount of upsetting cannot be reliably evaluated from the data on the amount of movement of the movable column. The data obtained from digital transmitters is used solely for indication and not retained. Also, the upsettings of welding conditions, caused by short-circuits in the welding circuit at the final stage of fusion, may happen to be not recorded if they have occurred in time between successive transmitter interrogation cycles. Moreover, when a short-circuit coincides in time with a transmitter interrogation cycle, the short-circuit occurrence itself can be detected from the value of the welding current, but the amount of exceeding of the permissible duration of welding process interruption cannot be evaluated.
Further, this apparatus fails to monitor the running time of the welding and hence makes it impossible to evaluate the conformity of variation of the welding process varibles at preset time moments.